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Hi, I’m Thomas Letan (lthms) (he/him).

You read something which caught your attention? Don’t hesitate to shoot me an email.


Extensible Type-Safe Error Handling in Haskell (

Ever heard of “extensible effects?” By applying the same principle, but for error handling, the result is nice, type-safe API for Haskell, with a lot of GHC magic under the hood.

Monad Transformers are a Great Abstraction (

Monads are hard to get right, monad transformers are harder. Yet, they remain a very powerful abstraction.